When a neuron fires, its voltage reverses for a very short time, about a thousandth of a second, he explained. “This brief spike in voltage travels down the neuron and then activates other neurons downstream. Our protein is sitting in the [outside] membrane of the neurons, so as that pulse washes over the proteins, they light up, giving us an image of the neurons as they fire.”

We can now see how these signals spread through the neuronal network, said Cohen.